The present invention relates generally to equipment for testing physical characteristics of materials, and more specifically, to a fretting fixture for a test machine that subjects a sample of material to high-cycle stress.
One type of stress-related fatigue damage that may occur in materials is xe2x80x9cfrettingxe2x80x9d. It occurs on a load-bearing contact surface between two pieces of mating material. Conventional fatigue testing has been empirically based without providing quantitative experimental information on the forces that result in fretting.
U.S. Pat. No. 6,023,980, to Thomas E. Owen, et al., entitled xe2x80x9cHigh-Cycle Fatigue Test Machinexe2x80x9d, describes a test machine that may be used to statically and dynamically load a test specimen in a manner that introduces controlled cyclic fatigue forces at high vibrational rates. A conventional use of this test machine is with a single specimen piece inserted between two actuators of the test machine, but this method of testing, by itself and without the supplemental fretting fixture invention described herein, does not introduce fretting forces in the specimen. However, by changing the method of excitation of this test machine, the static and dynamic forces applied to the specimen are modified and, additionally, the test specimen is made to undergo oscillatory translational motions along its length axis. As a result of these modified forces and translational motions and with the fretting fixture described herein attached to the specimen, the combined arrangement is capable of introducing fretting forces and fretting fatigue in the specimen.
The test machine described in U.S. Pat. No. 6,023,980 is recognized to have important advantages in imparting the desired controlled static and dynamic forces and translational motions to test specimens. However, by means of adaptations of the dynamic actuator component described in U.S. Pat. No. 6,023,980 in combination with the fretting fixture described herein, similar fretting testing results may be obtained using commercial materials testing machines capable of applying static tensile loading to the test specimen. In particular, a second test machine arrangement is one in which a specimen holder assembly and a dynamic actuator assembly are connected in tandem to form a tensile-loading column suitable for mounting in a conventional static-loading materials testing machine. When so mounted, the column may be placed in tension to produce a desired static tensile stress in the specimen. The dynamic actuator may then be excited to produce axially oriented mechanical vibration resonances in the column, with the principal resonance frequency governed by the compliance of the test specimen and combination of masses and compliances associated with the dynamic actuator assembly and the tensile-loading column components. When the dynamic actuator is caused to vibrate at the principal columnar resonance frequency, the specimen will undergo oscillatory translational motions. With the fretting fixture attached to the specimen, fretting fatigue effects may be induced in the specimen. The size and materials of the tensile-loading column and test specimen may be selected to cause principal mechanical resonances in the range of 1000-3000 Hz. The specimen holder assembly of this columnar testing module may be configured to accept the fretting fixture described herein without substantial modification.
One aspect of the invention is a fretting fixture for a test machine that induces high-cycle fatigue effects in prepared material test specimens. The test machine is assumed to be of a type that imparts both static tensile loading and dynamic oscillatory translational motions to a test specimen along its length axis. The fretting fixture is attached to the specimen and is freely supported on the specimen.
More specifically, the fretting fixture has a fretting piece that is placed against the test specimen. The combination of the fretting piece and the test specimen is positioned between two inertial masses comprising part of the fretting fixture and its clamping frame. That entire assembly is forcibly clamped onto and supported by the specimen.
An advantage of the invention is that it permits detailed and controlled study of material fretting effects during long-term cyclic vibration tests. Specifically, the fretting fixture permits accurate experimental simulation of fretting on material test specimens and accurate quantitative measurement of the applied static loading force and applied dynamic shear loading force that, in combination, induce the fretting effects. The forces are applied between two material surfaces, namely, the surface of the fretting piece and the surface of the test specimen, and are a result of oscillatory translation vibrations of the specimen at a frequency typically in the range of 1000-3000 Hz. The forces between the two surfaces are more clearly described as inertial reactions of the masses comprising part of the fretting fixture in response to the specimen oscillatory translation motions.
Another advantage of the invention is that the oscillatory translational displacement of the specimen relative to the fretting piece is measurable by means of sensors attached to the specimen grips and these motions may be controlled in amplitude by adjusting the excitation energy applied to the dynamic force actuators. The frequency of translational vibration and displacement can be made to be in the range of 1000-3000 Hz, depending on the physical design of the test machine, as in the case of the machine described in U.S. Pat. No. 6,023,980, or the design of the columnar testing module, as briefly described above, designed for use with commercial materials testing machines. This operating frequency range is a range in which fretting damage effects between mating materials are well known to occur. Likewise, force and acceleration sensors attached to the fretting fixture provide a quantitative measure of the fretting fixture static clamping force on the specimen and the dynamic reaction force between the fretting piece and the specimen.